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Ulcerative Colitis (UC) is a chronic inflammatory bowel disease located in the mucosa of the large bowel. UC often affects young adults between 15 and 40 years of age with no pre-dominant sex. Over time, incidence rates are steadily increasing and the cause of the disease remains unknown. Symptoms are general discomfort and bloody diarrhea. UC is diagnosed by endoscopic examination of the large bowel, where different hallmarks are found. It is of great importance that attacks/relapses are treated medically, as flares may cause death due to inflammatory destruction of the mucosa and perforation of the colon leading to extreme infection of the abdominal cavity. UC often affects the social life of the patients, as they feel that they must be in the immediate vicinity of toilets. Therefore, many patients prefer to stay at home during active disease. For society, UC is a costly disease due to patients reporting in sick and expensive medications. When medical treatment fails, UC patients must undergo surgery and have their colon removed (colectomy). This PhD project focused on the immune system of the body. Specifically, we looked into T cells (the chairmen of the immune system) that we believe play an important role in disease activity. When T cells are activated in inflammatory diseases, they produce several signaling substances (cytokines) that attract and activate the other parts of the immune system. T cells regulate their effector functions through calcium regulation. Upon activation, calcium is released from intracellular stores, which causes calcium channels to be embedded in the cell membrane (CRAC channels). As long as the T cells are stimulated, the two potassium channels KV1.3 and KCa3.1 maintain the driving force for calcium influx, thus keeping the T cells activated. Our aims were to investigate whether the two potassium channels KV1.3 and KCa3.1 were upregulated in mucosal biopsies from patients with active UC and whether there were correlations between the expression of the channels and the disease severity assessed by endoscopic and histological evaluation. Moreover, we used a rat colitis model (dextran sodium sulphate-induced) to examine the effect of pharmacological inhibition of KV1.3 and KCa3.1 on inflammation. We found that the expression of T cell potassium channel, KV1.3, was increased in active UC and a higher expression correlated well with both the endoscopic and the histological degree of inflammation. This suggests KV1.3 to be involved in the inflammatory process of UC. We did not find an increase of the other potassium channel, KCa3.1, at the gene expression level, but the channels were definitely present in the infiltrating T cells as examined by immunostaining. Preliminary gene expression data showed similar changes of gene expression in biopsies from Crohns disease (CD) patients. In addition, we conducted first pilot studies investigating whether pharmacological blockade of the channels ameliorates colitis in the rat DSS-model. We found a tendency towards less endoscopic inflammation in the acute phase (at day 7 and 10). However, at study termination, the improvement of inflammation failed to reach a significant level, presumably because of insufficient compound absorption from the intestine (based on low plasma concentration and previously reported amelioration of colitis by inhibiting KCa3.1). Based on these findings in our target identification study, it is suggested that both KV1.3 and KCa3.1 play a role in the inflammation of UC and possibly of CD and represent new pharmacological targets.